Modern Vespa

Hello everybody,
Another “ my vintage Vespa (1979 P200E) won’t start” post here.

From the beginning:
1. The headlight started going out at seemingly random intervals. It had been slightly damaged and cracked during a spill last winter. Replaced headlamp and checked all wires in the headset. Everything checked out good. Installed, and back in action.
2. Then it wouldn’t start. Checked all the normal things (fuel tap on, kill switch on, gas and oil full, pulled plug and had good spark,etc)
Got it to start, rode it a ways, and it died. Won’t start.
3. Got it to run for a while again with new plug, started hesitating and sputtering, particularly in fourth gear, but eventually even in second and third. It was getting extremely flooded even when the fuel tap was off. Checked fuel tap, all good.
4. Discovered that the rotary pad was toast (fuel flowing in unrestricted). Also discovered that that CDI was a bit shotty.
5. Got a gently used CDI and installed. Got rid of oil injection after having it for 25 years by upgrading to a brand new dellorto reed valve carb and installed.
6. Started well. Rode it home. Parked it for about three hours, and for two days now, it won’t start again. I tested all the things listed above. I rapid bunny kick it, change the plug, leave fuel tap on for a while first, etc etc. only thing that I haven’t tried yet is changing the plug gap.
I’m pretty darn frustrated at this point. Any advice would be greatly welcomed, as I am currently almost stumped. Thank you!

It's probably the green wire that the kill/key switch use to turn off the bike shorting out, the insulation on them is made of cheese or something (it fails on every bike of this era).

P200 green wire repair advice

Check timing and compression. Plug should be .025". Plug cap ? Or did the replacement come with its own ?
I don't think the pad is much of a seal against fuel and is usually checked with 2-stroke oil. 20 minutes from filling the space, I believe. I don't think you can check the pad visually from outside but maybe the crank web would show some scratching or build-up on the leading edge ?
Not sure what you mean by a reed valve carb. Do you mean a non-injected one ?
Is the plug wet after a few kicks of not starting ?
Disconnect kill wire ?

The hesitating in 4th is a sign. Usually a sign of the main oil seal letting air in. Eventually turning into starting issues when cold. Before getting serious.
Check the gear oil level, if you didn't already.

The rotary pad is surprisingly robust…

Scratches, gouges, chunks missing, it’ll still run… of course it’ll be running really badly, but you should be able to start…

I think I’ve mentioned it before on another post, but I’ve seen a 1mm gap between the crank and cheek on the clutch seal side and it still runs… a ton of blowback but still started

Thank you all for the advice.
Turns out that the bike starts a lot differently now that I switched from injection to pre mix with the Dellorto carb.
For the past 23 years that I have had the bike, it liked fuel tap on for a couple minutes followed by full throttle, no choke and a good hard kick. Now, it likes virtually no gas (throttle) at all, choke all the way out, and some rapid short kicks. If it doesn’t start on the first few kicks, I immediately turn off the valve and kick again until it starts. The reed valve carb stores way more gas, so is much more prone to flooding. I also reduced the plug gap from 0.5, to about 0.2, which seemed to help. Still running a B7ES.
Now we are up and riding. It turns out it was just learning about the new carb for me. I had gotten very very used to the original carb over the past 23 years.

What Delorto carb did you put on it ?

Educated guess, but I imagine your idle jet is a little lean... should start 1-2 kicks with the choke out... Reed or rotary setup

You could set it up to have a bigger idle jet and not need the choke to start, but you'll be spluttering oil and be pretty smokey at traffic lights.

There's so much potentially going on here that I almost don't know where to start.

You can search for more detail on how to run all these checks in this forum, so I'm going to skip the details. Plus I'm doing this on my phone because I hate myself.

First, if you installed a side draft carb with a reed manifold, which one?

I know you said you have spark, but I'll second pulling the kill switch wire off the CDI to reduce likelihood it's just that. Plus it's like a ten seconds to check.

Next, I'd try a new plug in case it's fouled. While you have the plug out, take a moment to inspect the piston crown and bore for any pitting (detonation) or scratches (seizing). If you can get pictures of the spark plug and the dome of the piston, those will tell us a lot.

After that, I'd re-confirm that I had spark by kicking the bike with the plug grounded to the motor.

I'd also check compression before putting the plug back in. You need about 90 PSI of cold compression for the motor to start.

Now, it's time to install the new plug, spray some starting fluid in that carb and see if it starts and runs for a few seconds. If it does, it's a carb or mixture issue. If doesn't, then you don't actually have spark and compression and need to run through your tests again.

Now I don't know what's actually going on, dive we don't have any answers back on the tests I listed advice, but my fear is that you have an air leak and the carb is too lean and you've jacked up the piston. That may be repairable, depending on the scale of it, but if you don't fix the root cause (the air leak(s) and carb setup), it'll just happen again, probably in a worse location than at home.

You'll want to pressure test the motor once it's buttoned up and you think any air leaks are fixed. A motor with an air leak will never really run right and always be at risk of seizing.

Definitely take and post those pictures. You'll be amazed at this forum's ability to troubleshoot off pictures.

Good luck!